1. Field of the Invention
The present invention relates to a powder for a dust core to be employed for a magnetic core of a transformer, an inductance and the like, a magnetic core for a motor, and other electronic parts and also relates to a dust core.
2.Prior Art
Recently, electric and electronic appliances have been miniaturized and along with the miniaturization, a dust core with a miniaturized size and a high efficiency has been required. As a magnetic material for a powder for dust core, a ferrite powder and a ferromagnetic powder are used. The ferromagnetic powder has a high saturation magnetic flux density as compared with the ferrite powder, so that the ferromagnetic powder is advantageous to enable a magnetic core to be miniaturized, however owing to a low electric resistance, it has a disadvantage that the eddy-current loss is increased. In order to lower the eddy-current loss as much as possible, an insulating film is formed on the surface of a ferromagnetic powder particle.
Other than that, in order to miniaturize the magnetic core, it is required that the saturation magnetic flux density is high and especially that a magnetic permeability in a high magnetic field of superimposed direct current is excellent and if the direct current superimposition high magnetic field property is excellent, the magnetic core can be miniaturized. That is since the operating magnetic field is defined as the electric current divided by the magnetic path length and therefore, if the magnetic core is miniaturized to shorten the magnetic path length, the operating magnetic field is transferred to the high magnetic field side. Even if the operating magnetic field is transferred to the high magnetic field side, a high inductance is obtained to make miniaturization possible, if the magnetic permeability in a high magnetic field of superimposed direct current is excellent and the magnetic permeability is high.
Further, other than the above, an inductor corresponding to a high current is required. In this case, also, if a magnetic core is excellent in the magnetic permeability in a high magnetic field of superimposed direct current, even in case that electric current is increased and the operation magnetic field is transferred to the high magnetic field side, the magnetic core can deal with the matter. Further, if the magnetic permeability in a high magnetic field of superimposed direct current is excellent and the magnetic permeability in a high magnetic field is not abruptly decreased, the number of turns of windings in, for example, an inductor can be increased and the inductance of the inductor is proportional to the square of the number of the turns of windings and therefore, further miniaturization is made possible.
However, even if dust core with which a magnetic core can be miniaturized is obtained, the size precision of the magnetic core becomes an important factor. In this case, particularly, it is required that the size alteration (hereinafter referred as to xe2x80x9cspring backxe2x80x9d) in the case of separation from a mold after molding is slight. Especially, if the magnetic core has a complicated shape, owing to the different molding pressure in respective parts, the degrees of the spring back differ and it is made difficult that the magnetic core is molded with a size high precision.
Before now, in order to mold a magnetic core with a high size precision, a lubricant is added to a ferromagnetic powder. For example, JP-A-12-30925 and JP-A-12-30924 disclose compacted powder magnetic cores with a high magnetic permeability, a low core loss and high mechanical strength as well and produced by mixing an atomized powder of a soft magnetic alloy such as a Fe-based soft magnetic alloy powder, e.g. an atomized powder of a Fexe2x80x94Sixe2x80x94Al based soft magnetic alloy having the average value of LL/LS ratio from 1.0 to 3.5, wherein LL denotes the length of the main axis and LS denotes the length of minor axis in the case of two-dimensional observation of the particle shape of the powder, with silicone resin, compacting and molding the resultant mixture, heating the compacted molded body at 600 to 900xc2x0 C., and after that, immersing the obtained compacted powder molded body in liquid resin, and curing the resin.
JP-A-11-195520 discloses a compacted powder core with a high magnetic flux density, a low coercive force, a low core loss and a high mechanical strength and a ferromagnetic powder for the core, and a production method of the compacted core which is produced, according to the disclosure, by using a ferromagnetic powder for a compacted powder core composed of a ferromagnetic powder, titanium oxide sol and/or zirconium oxide sol, and phenol resin, pressurizing and molding the ferromagnetic powder, and then heating the molded body at 500 to 800xc2x0 C.
JP-A-10-335128 discloses a compacted powder core using a ferromagnetic powder for a compacted powder core containing a ferromagnetic powder and 0.1 to 10% by volume of titanium oxide sol and/or zirconium oxide sol. Consequently, the disclosure actualizes a compacted powder core with a high magnetic flux density, a low coercive force, a low core loss and a high mechanical strength and a ferromagnetic powder for the core, and a production method of the compacted core.
JP-A-9-260126 proposes to actualize a compacted powder core having a high magnetic flux density, a low coercive force, and a low core loss, especially such excellent properties in a range of 50 to 10,000 Hz frequency and capable of replacing for a laminated silicon steel plate core in terms of intrinsic properties and discloses a compacted powder core containing 0.03 to 0.1% by weight of Si, 15 to 210 ppm of Ti, 300 to 2500 ppm of oxygen, and an iron powder with the particle size of 75 to 200 xcexcm and obtained by mixing an iron powder with the particle size of 75 to 200 xcexcm, silica sol in 0.015 to 0.15% by weight to the iron powder on the bases of solid matter, silicone resin in 0.05 to 0.5% by weight to the iron powder, and an organotitanium in 10 to 50% by weight to the silicone resin, hardening the powder mixture at 50 to 250xc2x0 C., molding the powder, and then annealing the molded body at 550 to 650xc2x0 C. in an inert gas atmosphere.
JP-A-9-170001 discloses a production method characterized as follows. A powder mixture of a soft magnetic iron powder, a heat resistant powder, an alkaline earth metal carbonate powder is heated and then the heat resistant powder and an alkaline earth metal oxide powder, which is a decomposition product of the alkaline earth metal carbonate powder, are separated from the resultant powder mixture. The powder mixture contains the alkaline earth metal carbonate powder in 0.5 to 5% by weight to the soft magnetic iron powder. The heating treatment is carried out in hydrogen/nitrogen mixture atmosphere or pure nitrogen atmosphere. Consequently, the patent proposes a compacted powder core with a high saturation magnetization and a low coercive force and possible to replace for a laminated silicon steel plate core and a soft magnetic iron powder to be used for producing the core at a low cost.
JP-A-8-45724 discloses a compacted powder core of a Fexe2x80x94P alloy powder containing 0.5 to 1.5% by weight of P and a compacted powder core using an organotitanium together with silicone resin as a binder. Consequently, the patent provides a compacted powder core with a low coercive force, a low core loss, a high saturation magnetization, and improved mechanical strength as well.
JP-A-8-37107 discloses a compacted powder core which is a core produced by compacting a ferromagnetic powder and an insulating agent and then annealing the compacted powder, wherein the ferromagnetic powder is an approximately spherical ferromagnetic metal particle containing Fe, Al, and Si in order to provide an economical compacted powder core with a low core loss and a compacted powder core with a low core loss and high mechanical strength.
Further, JP-A-7-254522 discloses a production method comprising a primary mixing step of mixing a ferromagnetic powder and silicone resin, a primary heating step of heating a primary mixture obtained in the primary mixing step in non-oxidative atmosphere, a secondary mixing step of mixing silicone resin and the primary mixture, a secondary heating step of heating a secondary mixture obtained in the secondary mixing step at a temperature lower then the treatment temperature in the primary heating step, a molding step, and an annealing step in this order. Consequently, a compacted powder core produced by pressurizing and molding the ferromagnetic powder is provided with improved magnetic permeability and its frequency properties and increased mechanical strength as well.
Further, these patents disclose a stearic acid metal salt including aluminum stearate can be employed.
Further, JP-A-12-49008 discloses a ferromagnetic powder for dust cores containing at least one stearic acid metal salt selected from magnesium stearate, calcium stearate, strontium stearate, and barium stearate.
Any of the above described patents such as JP-A-12-30925 and the like provide a ferromagnetic powder for dust cores with magnetic properties such as a high saturation magnetic flux density, a low core loss, and a high magnetic permeability or the like. However, none of the patents discloses a ferromagnetic powder for dust cores with excellent mechanical properties such as capability to increase the strength of the molded body and to lower the spring back degree after separation from a mold.
The present invention therefore has a object to provide a powder for dust cores capable of improving magnetic properties such as magnetic permeability in a dust core and improving mechanical properties such as size precision of the molded dust core and radial crushing strength and to provide dust cores using the powder.
In order to achieve the above described object, a first aspect of the present invention provides a powder for dust cores containing a ferromagnetic powder, an insulating material containing silicone resin and/or phenol resin, and a lubricant, wherein the lubricant contains aluminum stearate.
A second aspect of the present invention provides the powder for dust cores of the first aspect, wherein the lubricant is aluminum stearate with the metal content of 4% by weight or more.
A third aspect of the present invention provides a dust core produced by mixing a ferromagnetic powder, an insulating material containing silicone resin and/or phenol resin, and a lubricant and molding the resultant mixture, wherein the lubricant contains aluminum stearate.
A fourth aspect of the present invention provides the dust core of the third aspect, wherein the lubricant is aluminum stearate with the metal content of 4% by weight or more.